Temporal lobe epilepsy is common and difficult to treat. Our long-range research goal is to help reveal mechanisms of temporal lobe epilepsy and develop anti-epileptogenic strategies. During the last funding period we discovered rapamycin suppresses granule cell axon (mossy fiber) sprouting and found that despite substantial reductions, spontaneous seizure frequency was unchanged in a mouse model of temporal lobe epilepsy. These findings raise doubts about the role of mossy fiber sprouting in epileptogenesis. However, scattered clues in the literature and preliminary findings suggest another recurrent, excitatory circuit in the dentate gyrus developed despite suppression of mossy fiber sprouting. The goal of the next funding period is to test the hypothesis that in a rat model of temporal lobe epilepsy surviving mossy cells function as aberrant network hubs, establish an over-developed positive-feedback circuit within the dentate gyrus, and contribute to the generation of spontaneous seizures. We propose 3 specific aims. Aim 1 is to test the hypothesis that surviving mossy cells enlarge, elongate dendritic processes, and receive more excitatory synaptic input in epileptic pilocarpine-treated rats. Hippocampal slices, whole-cell patch recording, and biocytin labeling will be used. Aim 2 is to test the hypothesis that surviving mossy cells sprout axon collaterals and form new synapses with granule cells in epileptic pilocarpine-treated rats. In vivo intracellular biocytin-labeling and electron microscopy will be used. Aim 3 is to perform 3 experiments to test the hypothesis that surviving mossy cells contribute to seizure generation. Unit recordings will be obtained from mossy cells in epileptic pilocarpine- treated rats as they experience spontaneous seizures. Seizure frequency and numbers of surviving mossy cells will be measured to determine whether there is a correlation. And, we will test whether epilepsy-related changes in mossy cell circuitry are resistant to rapamycin treatment. Achieving these aims will clarify the role of surviving hilar mossy cells in temporal lobe epilepsy. If mossy cells do play an epileptogenic role, then they may offer new therapeutic targets to dampen excitability and provide better seizure control for patients.